1. Technical Field
This disclosure relates to the use of novel catalysts in various chemical reactions. Specifically, the disclosure is directed to N-Heterocyclic Carbene-Amido Palladium(II) catalysts and use of these catalysts in organic synthesis.
2. Description of Related Art
Catalysts are one of the most important classes of compounds in nature. Catalysts have the capability of significantly increasing reaction rates of a variety of compounds. Estimates are that 90% of all commercially produced chemical products involve catalysts at some stage in the process of their manufacture.
Palladium catalyzed coupling reactions are extremely powerful and versatile synthetic methods for carbon-carbon and carbon-heteroatom bond formation. N-heterocyclic carbenes (NHCs) have been employed successfully as ligands to generate a variety of palladium complexes due to their enhanced chemical properties and the strong Pd-NHC σ-bonding which contributes to the high stability of the active species compared with traditional phosphane ligands.
Lee and Hartwig have prepared monodentate NHC ligands with a bulky chiral terpene group attached to the nitrogen atoms for use in asymmetric intramolecular α-arylation. However, these enantioselective transformations employing a monodentate ligand can be cumbersome since various inactive palladium-ligand complexes, such as a trans-conformation, may be prepared.
NHC ligands are also known to have enhanced σ-donor coordination which allow for stable catalysts for C—H activation. However, most examples refer to intramolecular processes, as intermolecular or catalytic examples are rare. In particular, aqueous conditions for organometallic complexes are not developed because of the vulnerability of metal-carbon bonds, thereby limiting their roles in innocent auxiliary ligand sets or transient ligands in catalytic reactions. In recent years, a substantial number of metal complexes that are able to selectively activate C—H bonds under mild conditions have been discovered. However, practical catalysts for C—H bond functionalization have been elusive, partially due to the fact that the C—H bond activation is often inhibited by water or by the alcohol product eliminated from the metal complex during the reaction.